The present invention relates to an addition curing system, and more particularly the present invention relates to heat curable organo polysiloxane compositions having a low viscosity in the uncured state and high physical strength in the cured state.
An example of a platinum catalyzed curable organopolysiloxane composition based on the use of a vinyl containing diorganopolysiloxane and a silicon hydride utilizing a platinum catalyst is shown by Modic, U.S. Pat. No. 3,436,366, assigned to the same assignee as the present invention. The composition of Modic utilizes a vinyl containing diorganopolysiloxane which is incorporated into a silicon hydride-olefin-platinum catalyst composition.
Another SiH-olefin system is shown by Jeram et al, U.S. Pat. No. 3,957,713, which utilizes in addition to the combination of a vinyl containing diorganopolysiloxane, a silicon hydride and a platinum catalyst, a low viscosity organopolysiloxane fluid which is terminated at one end with a triorganosiloxy group and the other end a vinyl diorganosiloxy group. Jeram et al also teaches that a silica filler can be utilized, for example, fume silica or precipitated silica which are preferably treated with cyclic polysiloxane or silazane to provide high strength compositions in the cured state.
Additional compositions are shown by Jeram, U.S. Pat. No. 4,041,010, directed to fluorosilicone compositions, based on the use of a fluorosilicone, a vinyl fluorosilicone resin, a silicon hydride cross-linking agent and a platinum catalyst which are particularly useful in liquid injection molding applications. In addition, U.S. Pat. No. 4,029,629, Jeram, teaches the use of a reinforcing filler in combination with the aforedescribed silicone composition.
Experience has shown that liquid injection molding apparatus requires that the uncured organopolysiloxane composition have a viscosity of about 10,000 to 25,000 centipoises at 25.degree. C. so that the organopolysiloxane can be readily injected into the molding chamber and thereafter the composition has to cure at a sufficiently rapid rate. It is also known by those skilled in the art that platinum catalysts, for example, Lamoreaux U.S. Pat. No. 3,220,472 and Karstedt, U.S. Pat. No. 3,715,334 are directed to valuable platinum complexes which can be utilized to effect addition between SiH and vinyl containing diorganopolysiloxane. It is further known that use of such platinum catalyst with mixtures of silicon hydride and vinyl containing diorganopolysiloxane do not possess sufficient shelf stability at 25.degree. C. to be useful as one component curable organopolysiloxane compositions in the form of a mixture of the platinum catalyst, the silicon hydride and the vinyl containing organopolysiloxane. As a result, inhibitors, for example, Bobear, U.S. Pat. No. 4,061,609, have been development in the form of hydroperoxy compounds. However, inhibitors often have been found to interfere with the functioning of the catalyst, particularly in instances where the inhibitor is employed with the platinum catalyst and the vinyl containing diorganopolysiloxane as part of a two component mixture which can be subjected to elevated temperatures prior to combining the aforementioned ingredients with the silicon hydride component under liquid injection molding conditions.
The present invention is based on the discovery that a halogenated tetameric platinum complex having the formula, EQU [Pt(CH.sub.3).sub.3 (X)].sub.4 ( 1)
where X is a halogen radical, for example, chloro, bromo, iodo, etc., can be utilized in combination with one component silicon hydride containing siloxane and vinyl polydiorganosiloxane liquid mixtures without the use of an inhibitor to produce heat curable organopolysiloxane formulations which can be used in a variety of applications to make high strength cured organopolysiloxanes after an extended shelf period as a liquid mixture. The one component liquid mixtures, for example, can be used with or without reinforcing silica fillers in various applications, such as liquid injection molding. In addition, the platinum catalyst of formula (1) can be utilized in combination vinyl containing polydiorganosiloxane at elevated temperatures in absence of an inhibitor and thereafter combined with a silicon hydride as part of a two component liquid injection molding formulation. Unlike the platinum catalysts of the prior art, which often react with an inhibitor, rendering the platinum catalyst less effective as part of a two component liquid injection molding organopolysiloxane formulation, the tetameric platinum catalysts of formula (1) are not subject to prior "poisoning" because these halogenated tetrameric complexes of platinum do not require an inhibitor. In addition, inhibitors often have been found to adversely affect the physical properties of the cured organopolysiloxane.